The architecture of large Internet Protocol (IP) networks is based on routers that connect to a Wavelength-Division Multiplexing (WDM) or Dense Wavelength-Division Multiplexing (DWDM) system directly via transponders. In such systems, the DWDM layer provides low cost transport, but does not provide protection against signal failures. The routers provide all such protection via mechanisms such as Internal Gateway Protocol convergence or Multiprotocol Label Switching Fast Re-Route. These protection mechanisms can be initiated proactively when a link on a path in the network degrades. But, such protection is at the router level, i.e. at the IP layer, and that the IP layer needs extra capacity to be available necessary for such protection mechanisms, resulting in a higher overall cost of the network.
Another signal failure protection mechanism is multi-layer restoration. Multi-layer restoration, enables restoring full capacity of the IP layer, by switching the path of the affected IP links in the DWDM layer. If the IP layer is not involved in coordinating the restoration process, and it happens spontaneously by the optical layer, the mechanism is called optical restoration.